1. Field of Invention
This invention relates to electro-optical switches and more specifically to a system for controlling the voltage switching levels of an electro-optical switch array.
2. Description of the Related Art
Optical switch arrays are used in many different applications when it is desired to multiplex an optical signal along multiple paths. For example, it is known to use optical switch arrays in multiple-ring fiber-optic gyro (FOG) systems to direct the optical signal emitted from a single source to multiple rings and from the rings to a detector. Such arrays are reconfigured periodically by the selective application of electrical signals.
A FOG system is used to sense rotation of a vehicle (e.g., a spacecraft) about one or more axes of rotation, and outputs from the FOG system are used to provide navigation and flight control information for the vehicle. A typical FOG includes a laser source providing an optical signal and a multi-turn coil of optical fiber referred to as a fiber-optic ring. The optical signal is first applied to an optical beam splitter/combiner which provides two identical optical output signals, each of which is applied to one end of the fiber-optic ring. The two signals travel through the ring in opposite directions and are recombined at the beam splitter/combiner. Any rotation of the fiber-optic ring about its wound or longitudinal axis will result in a phase shift of the signals traveling through the ring. This phase shift is known as the Sagnac effect phase shift and is detected by analysis of the recombined signal from the splitter/combiner that is applied to an output detector.
A particular implementation of a FOG incorporating an optical switch array is described in U.S. Pat. No. 4,828,389 to Gubbins et al. issued May 9, 1989, which is incorporated by reference herein. The FOG described in that patent comprises three fiber-optic rings oriented along orthogonal axes representative of axes of rotation of the vehicle in which the FOG is used. In that arrangement, light from a single optical source is transmitted to a multi-port optical switch array which transmits the signal to the three separate rings, via separate beam splitters/combiners, on a time-shared basis. The optical switch array further functions to transmit the recombined signals on a time-shared basis from the three rings to a single optical detector.
The optical switch array described in the above-noted patent comprises three switching stages interconnected by optical channel waveguides. Each switching stage comprises a bi-directional electro-optical switch having two pairs of optical ports and two pairs of electrodes by which control voltages may be applied to the switch. The switches may be fabricated from a crystalline material, such as lithium niobate (LiNbO.sub.3), the index of refraction of which changes as a voltage signal is applied to the crystal. The optical channel waveguides may be formed in the crystalline material by indifusion of a dopant such as titanium. By the proper application of appropriate voltage control signals to the electrodes, each switch may be set to a "cross" state in which a light beam is deflected in the switch, and a "bar" state in which the light beam is passed through the element without deflection. By selective application of control signals, controlling the cross and bar states of the individual switches, it is possible to transmit optical signals between optical ports of the switching array in each of a plurality of time slots. In the arrangement described in the Gubbins et al. patent, two separate optical paths are established in the switch array, in each time slot. One path allows for the transmission of a signal from a laser source to one of the fiber-optic rings and the other path allows for the transmission of a recombined signal from another of the rings to the output detector, in the same time slot. Since each stage of the switch array comprises two electrode pairs, six separate voltages must be applied to a three-stage array in each time slot. Since each switch may be placed in either the cross state or bar state, twelve different control voltages must be available for control of the three-stage switch array.
The application of voltage signals to the electrodes results in the establishment of electric fields which create the bar and cross states within the switch. The magnitude of an applied voltage at which an electro-optical switch assumes the bar state or the cross state is a function of the physical characteristics of the crystal element and may change with changes in environmental conditions, such as temperature, and other changing conditions such as charge migration and device life. It has been recognized that the application of a voltage signal of improper magnitude for either the cross state or the bar state may cause an optical signal to be partially blocked or misdirected. This phenomenon, referred to as "leakage," may result in a reduced magnitude or erroneous output signal from the switch and ultimately in erroneous flight control information. It is therefore desireable to minimize such leakage.